Answer
$4C $
Work Step by Step
$\log_{b}81=\qquad $... recognize $81$ as a power of $3$
$=\log_{b}3^{4}\quad $...apply the Power Rule: $\quad \log_{b}(M^{p})=p\cdot\log_{b}\mathrm{M}$
$=4\log_{b}3\quad $... use the definitions of A and C
$=4C $
